This is billed as the Brexit election – the opportunity to refight the battles of a year ago over the biggest political question in a generation.

But it could yet turn into an election fraud election – with a cunning plan hatched by David Cameron’s Remainer CCHQ more than two years ago coming back to haunt Theresa May and her merry band of Brexiteers.

The date everyone is watching is 8 June – polling day. But 20 May could be when this election is turned on its head, the deadline for the first of at least 14 current Tory MPs to find out if they are to be prosecuted for election fraud. The others, it might be up to 20, or even more, must learn their fate by early June.

They won’t be MPs by then of course. Parliament will have been dissolved by 3 May at the latest. But they could be forced to file their nomination papers by 11 May with an unresolved year-long police investigation hanging over their heads.

So we could see a string of what were once considered marginal Tory seats being defended by candidates who face being put on trial during the next parliament accused of being fraudulently elected to the current one.

That’s why Professor Justin Fisher, of Brunel University, said this week that the timing of June’s election could prove a “considerable miscalculation”.

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Since the Mirror broke the story that at least two dozen Tory MPs may have committed election fraud by failing to declare the costs of the help they received from the now notorious battlebuses, all the MPs and their party have denied wrongdoing.

At the last general election the Conservatives spent £15.6m, some £3.6m ahead of Labour and more than all the other parties combined. That was well short of the £19.4m national campaign limit.

Locally, things were much tighter. During the final six weeks of the election campaign, parliamentary candidates may spend no more than £8,700 plus 9p per voter in county constituencies and 6p per voter in borough seats. Typically this works out at £15,000 or less.

Many candidates ran close, particularly those in well-funded, target seats.

Tory HQ’s “40/40” strategy involved going all out on defending its 40 most losable seats and gaining the 40 most winnable.

A plan was hatched to send battlebuses packed with eager Tory activists to many of these seats to help with the final push. It was signed off by the Executive Senior Management Team chaired by Cameron’s old university friend and tennis partner Lord Feldman.

It was a hugely successful. In the euphoria of the surprise Conservative election victory, David Cameron told bus organiser Mark Clarke: “We quite simply could not have done it without you.”

But these battlebuses, part of Clarke’s Roadtrip2015, would come back to bite Cameron and his party time and again.

Four months after the election, young party activist Elliot Johnson took his own life, claiming he had been bullied by Clarke. Lurid allegations of sexual harassment and bullying on the buses followed and an official Tory probe identified a further 12 alleged victims. Clarke denies all the allegations.

I began investigating the Tory battlebuses at the start of February 2016.

The prolific tweets of those involved offered a snapshot of a typical day. There were a series of buses dispatched to different parts of the country in the final stages of the election campaign.

After arriving in a target seat, a picture was taken of the volunteers and often a Tory grandee sent to rally the troops – a Grant Shapps or a Sajid Javid or an Alan Duncan – posed in front of the bus with the local candidate.

So, on the morning of 27 April 2015, the battlebus rolled into Cannock Chase, in Staffordshire, a Tory seat being defended by local councillor Amanda Milling.

At 9.40am, the official Roadtrip2015 account tweeted: “Just touched down in Cannock Chase. Looking forward to a great day of campaigning for @amandamilling. #Battlebus2015”

Then it posted a picture of party chairman Patrick McLoughlin posing in front of the bus with Ms Milling and holding a large placard with her name and face on it.

During the afternoon, Roadtrip2015 reported that, “The lunch break is over in #cannockchase. #Battlebus2015 volunteers back out and about supporting @amandamilling this afternoon,” followed by, “Last session of an amazing day out campaigning in Cannock Chase. Great support on the doorsteps for @amandamilling”.

Ms Milling was delighted with their efforts, tweeting that evening: “Fantastic to have so many brilliant, enthusiastic campaigners out today supporting my local campaign – thank you @Roadtrip2015”.

The same pattern was repeated over the country during the following days.

The Mirror’s Nick Sommerlad

Theodora Dickinson, now Campaigns Director at Conservative Way Forward, joined the one of a series of battlebus trips to South Thanet, Kent, on April 26, where candidate Craig McKinlay was defending the Conservative seat from UKIP leader Nigel Farage.

Holding up leaflets promoting McKinlay, she tweeted: “Great day campaigning for @cmackinlay in South #Thanet with @Team2015! #VoteConservative #BetterFuture”.

A week later, she was in Derbyshire. She posted a picture of herself standing at a front door, holding a leaflet for Amber Valley MP Nigel Mills, adding: “Great to be speaking to voters in Ripley on behalf of @VoteNigelMills today! #Team2015 #VoteConservative”.

My Mirror colleague Nicola Bartlett and I found evidence this was happening on an industrial scale. One activist tweeted on May 4: “New #Battlebus2015 record! 1,719 conversations had with voters today in #Lincoln for @karlmccartney.”

So, had any of the candidates who received this help – the majority of whom went on to win their elections – declared any of the costs?

The national spending returns, where the battlebus costs had been declared, were already online and could be explored. But the returns for each candidate are only kept as paper copies in council offices by returning officers for two years before they are destroyed.

Failing to declare election spending by candidates and their agents is a criminal offence but there is a one-year time limit on investigations. Time was already running out.

More Mirror reporters from around the country helped collect spending returns from constituencies where the battlebuses had visited. I soon had more than 40 spending returns and only one recorded anything that looked like a battlebus expense. Anna Soubry’s local Beeston Conservative Club had invoiced £140 for “refreshments for Team 2015 activists”.

But there was no mention of the £400-a-day bus hire or the hotel bills for keeping 50 volunteers housed during the ten-day trips.

From the limited information available from the national spending returns, we erred on the side of caution and estimated the buses could have cost £2,000 a day.

Her spending return revealed that Soubry was within £894 of the local spending cap of £15,000 during the short campaign and had declared no transport or hotels costs.

She told the us: “I am aware that CCHQ campaigned across the UK for the return of a Conservative Government, including in seats such as mine. Such campaigning would be part of the national return and not within my local return.”

We splashed our investigation on 1 March 2016 and ran it over four pages inside. We named 20 winning Tory MPs who had received a visit from a battlebus but were within £2,000 of their spending limits.

Its political editor Michael Crick wrote a piece for our next edition calling on the authorities to investigate: “The Mirror has raised more serious questions about whether 24 Tory MPs failed to declare thousands of pounds bussing in scores of eager Tory activists to fight for marginal seats.

“If so, it casts even more doubt on the paper-thin majorities that gave the Tories their grip on power. These are all serious allegations growing day-by-day.”

But our expose fell between a rock and a hard place. Failing to declare candidate spending, an offence under the Representation of the People Act, was a matter for the police. Few detectives had any experience of such investigations. Professor Fisher believes there has been only one previous conviction.

The Electoral Commission told me at the time it only has powers to investigate national party spending. It had been asking for these to be extended to cover individual candidates but been rebuffed by David Cameron’s Government.

The Mirror provided the commission with copies of the tweets above, along with dozens more. But the commission initially decided there was “not sufficient evidence of any offence under the Political Parties, Elections and Referendums Act 2000 to pursue this matter further”.

It could only check whether the party’s national spending return was complete. The bus appeared to be on a national tour and many of the social media pictures showed people holding up national Tory party signs.

An official added: “Whether any of the spend should also have been captured as part of a candidates return (for example, the cost of signs including their name) is a matter for the candidate under the Representation of the People Act. As you know, any breaches of this legislation is a matter for the police to consider, not the Electoral Commission .. we would encourage you to contact the police directly if you wish to raise this matter with them – however, the information you provided to us does not meet the threshold for us to take any further action”.

It was later to admit that the social media images formed crucial evidence: “There were extensive social media posts from the time of the Battlebus2015 campaign activity that show activists from the coaches holding campaign material promoting individual candidates as opposed to the Party … the Commission is satisfied that a proportion of the reported spending was candidate campaign spending and should not have been included in the Party’s return. That proportion was also, as a result of this, not included in any relevant candidate’s campaign expenses return, casting doubt on the accuracy of those returns.”

Following a string of complaints to police and some excellent work by Channel 4 News reporting the undeclared hotel bills and the leaflets and scripts used by local candidates, with the clock ticking, the Electoral Commission eventually changed its mind and met with police and the CPS. The Police asked to see the evidence and their investigations began.

Last May, police forces began applying through the courts for one-off one-year extensions to the one-year time limit on their investigations. All got them. The CPS is now bound by the same deadline. We will learn the outcome in the coming weeks – just in time for the next general election.